Friction dampers developed by Dr. Avtar Pall have been widely used in the construction industry since the 1980s. The friction dampers are effective for seismic control of buildings, i.e. making the buildings more resistant to forces from earthquakes.
The designs of the friction damper are described in Canadian patent no. 1,150,474 and U.S. Pat. No. 4,409,765. The friction dampers are installed in the structure of the building and operate by converting seismic energy from earthquakes into friction/heat.
FIG. 1A illustrates a side elevation view of prior art friction damper 1. FIGS. 1B, 1C, 1D illustrate section views along the lines 1-1, 2-2, and 3-3 of FIG. 1A, respectively. The prior art friction damper 1 includes a pair of opposed gusset-side plates 4 that are spaced apart to define a slip channel 8 therebetween (see FIG. 1C). The proximal end region 12 of the gusset-side plates 4 are engageable with a gusset of a building structure. For example, and as illustrated, the proximal end region 12 include openings 16 for attachment to the gusset of the building structure.
The gusset-side plates 4 extend along and define a bracing axis 20. Accordingly, the slip channel 8 also extends along the bracing axis 20.
The cross-section at line 2-2, as illustrated in FIG. 1C corresponds to a first non-overlapping portion 24 of the friction damper 1 and the length of the slip channel 8 corresponding to this first non-overlapping portion 24 is unoccupied when the friction damper 1 is in its non-slip state, thereby defining a clear space within the slip channel 8.
The prior art friction damper 1 includes a brace-side plate 28. A proximal end region 32 of the brace-side plate 28 is engageable with a brace of the building structure. The brace-side plate 28 extends along, and is aligned with, the bracing axis 20. The brace-side plate 28 is also partially received within the slip channel 8.
The cross-section at line 3-3, as illustrated in FIG. 1D corresponds to a second non-overlapping portion 36 of the friction damper 1 and a length of the brace-side plate 28 is not overlapping with the gusset-side plates when the friction damper 1 is in its non-slip state.
The cross-section at line 1-1, as illustrated in FIG. 1B corresponds to an overlapping portion 40 of the friction damper 1 in which a length of the brace-side plate 28 overlaps with the gusset-side plates 4. Frictional engagements are formed between friction surfaces of the brace-side plate 28 with friction surfaces of the gusset-side plates 4. More particularly, a first outer friction surface 44 of the brace-side plate 28 forms a first frictional engagement with an inner surface 48 of a first of the gusset-side plates 4. A second outer friction surface 52 of the brace-side plate 28 forms a second frictional engagement with an inner friction surface 54 of a second of the gusset-side plates 4. The friction surfaces of the brace-side plate 28 and gusset-side plates 4 are treated to improve the frictional engagement.
As illustrated in FIG. 1B, a first clamping member 60 is disposed on the outer surface of the first of the gusset-side plates and a second clamping member 64 is disposed on the outer surface of the second of the gusset-side plates. Fasteners 68 extend through openings of the gusset-side plates 4, brace-side plate 28 and clamping members 60, 64 to fasten the clamping members 60, 64 together. The clamping by the fasteners 68 apply a normal force on the surfaces of the brace-side plate 28 and gusset-side plates 4, thereby causing the frictional engagements thereof.
In a non-slip state, as illustrated in FIG. 1A, frictional engagements of the friction surfaces of the brace-side plate 28 and the gusset-side plates 4 is maintained. The frictional force of the frictional engagements is sufficiently high to maintain the frictional engagement under seismic forces applied along the bracing axis 20 (compression or tension) within the building structure (i.e. when an earthquake is not occurring).
When a seismic force (compression or tension from an earthquake) applied along the bracing axis 20 is greater than the frictional force of the frictional engagement, the brace-side plate 28 and the gusset-side plates 4 slip relative to one another. FIG. 1E is a hysteresis loop showing the slip load from the slip length (length of travel of the plate 28 relative to plates 4). It will be appreciated that when the brace-side plate 28 and the gusset-side plates 4 slip relative to one another, the energy applied along the bracing axis 20 from the earthquake is converted to friction heat from the engagement between the surfaces 44, 52 of the brace-side plate 28 and surface 48, 54 of the gusset-side plates. This conversion of energy reduces the forces on other parts of the building structure, thereby improving building integrity during the earthquake.